The present invention relates to the preparation of polycrystalline diamond and more particularly to polycrystalline diamond possessing enhanced thermal conductivity.
High thermal conductivity diamond, such as high quality selected type II natural diamond, is characterized by a very high degree of purity and is reported to have a thermal conductivity at 25.degree. C. (298.degree. K) on the order of about 21 watt/cm .degree.K. Such high thermal conductivity diamond is useful, for example, as a heat sink material, such as in the backing of semi-conductors. Despite its high costs, type II natural diamond has been employed as a heat sink material because it has the highest thermal conductivity of diamonds. Conventionally-produced high pressure/high temperature (HP/HT) synthetic high quality, low nitrogen type II diamonds can be produced with a similarly high thermal conductivity. For the most part, diamonds prepared by low-pressure chemical vapor deposition (CVD) processes are not single crystal diamond and have materially lower thermal conductivities, typically on the order of 12 watts/cm .degree.K at about 300.degree. K (hereinafter sometimes referred to as "room temperature conductivity").
Since diamond is usually an electrical insulator, i.e. electrically non-conducting, heat is conducted by phonons. Anything that shortens the phonon mean free path (i.e. lattice vibration modes) degrades thermal conductivity. In 98% of natural diamonds (type Ia), nitrogen impurities scatter phonons. This reduces the mean free phonon path and, thus, the thermal conductivity, to near 8 watts/cm .degree.K. In polycrystalline diamond typical of that made by CVD processes, there are many defects, such as, for example, twins, grain boundaries, vacancies, and dislocations, that reduce the phonon mean free path. The thermal conductivity of CVD diamond is remarkable in one sense in that it is about 60% of the thermal conductivity of highly perfect diamond.
With respect to polycrystalline diamond (in film, compact, or other form), thermal conductivity is known to be affected by, for example, impurities, isotopic effects, and grain boundary scattering, to name just a few factors. In fact, grain boundary scattering has been believed to be dominant in the lower thermal conductivity of polycrystalline diamond compared to single crystal diamond. Enhancement of the thermal conductivity of polycrystalline diamond, then, is a need that yet exists in the art.